Just wired up an Automator quick action to convert video files to .mp3. I used the following shell script to do the actual conversion (mostly copied from this answer by Lukas Knuth).
/usr/local/bin/ffmpeg -vn -sn -dn -i "$1" -codec:a libmp3lame -qscale:a 4 "${1%.*}.mp3"
Now i have to do some work to fix up my blog-image-processing toolchain...